For reducing the pollutant component in exhaust gases (waste gases) of internal-combustion engines and firing installations, particularly small firing devices, the composition of the exhaust gas has to be monitored and the fuel component has to be varied by suitable control devices. In internal-combustion engines additionally exhaust gas treating devices are provided. In internal-combustion engines operating with varying load requirements, particularly in internal-combustion engines for automotive vehicles, the exhaust gas treating device has to be submitted to a follow-up regulation dependent upon the load requirement and the different contents of pollutants resulting therefrom, particularly nitrogen oxides and hydrocarbons.
To perform the above-outlined regulations, exhaust gas (waste gas) sensors are provided which, however, are exposed in an environment characterized by a changing gas composition, high temperature fluctuations and also high temperature gradients. The exhaust gas sensors, however, do not remain stable, because they have a limited operating range as concerns an upper temperature limit and are also exposed to a "drift", that is, because of the changing temperature and concentration their zero-point is affected.
Several methods for calibrating measuring devices are known:
German Offenlegungsschrift (application published without examination) 42 17 893 describes a method of calibrating gas sensors for smoke gas analyses. This method uses air as the reference or calibrating gas for calibrating the measuring value for determining an O.sub.2 -concentration.
German Offenlegungsschrift 39 39 166 describes a calibration of sensors by using electrochemical effusion cells. Here too, the object is to set the concentration signal with the aid of a calibrating device.
In German Offenlegungsschrift 35 46 409 for the calibration of chemical sensors a calibrating gas is admitted under pressure to the sensor surrounded by the liquid to be analyzed. The calibrating gas displaces the liquid at the sensor and by capturing the pressure and temperature values at the sensor the concentration of the components of the gas to be calibrated may be determined by computation and compared with the sensor signal.
In German Offenlegungsschrift 31 26 647 a quasi-electric calibration is described in which an electric resistance is used to expand in time the calibration of the measuring instrument by calibration gases with the aid of an alignment of the constant electric resistance with the simulated measuring values.
The above-outlined known methods in all instances presuppose the existence of a measuring value or a measuring value indication, that is, a value which is above the zero-point because only in such a case can a deviation be determined. The "wandering" or "drifting" of the zero point cannot be determined with the known methods and thus can also not be taken into account in a regulating device.
It is further feasible to detect the drift and also to achieve a stability of indication by taking into account the changing influences with the aid of different algorithms in the software of the regulating device. For such a proceeding, however, an independent reference system is not available which also has a certain stability as concerns aging.